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When soil calcium is high: High levels of soil calcium may be expected when there is substantial calcium carbonate bedrock from which the soil has weathered. Calcium levels can be high in agricultural settings from fertilizers. It's also not uncommon to find relatively high amounts in certain clays due to their heightened CEC.

  • For example, this source (and others) suggest that calcium levels would be expected to be higher in clay vs sand (with some impact from pH):

    Calcium in soil is generally correlated with the amount of sand versus clay in soil as well as soil pH. Sandy soils tend to have calcium levels at or below 1000 lbs/acre, whereas high clay soils often have levels above 2,500 lbs/acre, Soils with a good pH level also tend to have plenty of soil Ca.

My situation: I've found relatively high levels (1200-2000 ppm) of soil calcium (measured by Mehlich III extraction; Mehlich 1984) from fairly non-acidic (pH = 5.5-6.5) alluvial sandy soils.

  • the soil type: "Cartecay and Chewacla soils, 0 to 2 percent slopes, frequently flooded" (with perhaps mix of "White Store sandy loam, 10 to 25 percent slopes")

  • the site is in a forested urban patch in central North Carolina (USA) and is neighbored by a small 1st order stream that occasionally floods the site after elevated post-storm drainage

For reference, nearby high-clay (more acidic) soils in higher topographic positions tended to have lower calcium levels between 200-1000 ppm.

My question: what would be the source/cause of high calcium in sandy alluvial soils?

  • What role does the nearing-neutral pH have?

  • Are there likely "natural" (e.g., geologic) sources of heightened calcium in this case, or is this likely a phenomenon due to high calcium effluent from upstream non-point pollution sources (e.g., golf course, hospital, parking lots)?


Update:

I've also now noticed that the soil has many traits (e.g., high pH, Mg, Cu, Fe, Mn, and Zn) that could associate it as being a result of an ultramafic intrusion (known to occur in this general region). However, I know that qualitatively speaking, ultramafic intrusions would have a high Mg/Ca ratio. My soil's ratios (for both A and B horizons):

  • ppm_A: 350 ppm Mg / 1800 ppm Ca = 0.19

  • ppm_B: 315 ppm Mg / 1150 ppm Ca = 0.27

  • percent saturation A: 17% Mg / 50% Ca = 0.34

  • percent saturation B: 19% Mg / 38% Ca = 0.5

I'm not sure what constitutes "high" as I'm unsure of a baseline (i.e., is high much greater than a ratio of 1 or simply greater than a ratio of 0?).

Is it possible that this hypothesis holds any weight??

  • other issues: do ultramafic intrusions result in sandy alluvial soils?, here suggests there'd be high Na, of which my soil has very low amounts (1/50th of which the linked source suggests).
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